This invention relates to permanent magnet excited synchronous motors. More particularly, it relates to a rotor for such a motor in which high strength is achieved while flux leakage is minimized.
Centrifugal and magnetic forces in a motor tend to force the components of a spinning rotor away from its axial center line. Structures are therefore provided to support or contain these components under the action of such forces. However, these structures may tend to interfere with many properties of the rotor.
In many machines, especially those of small size which operate at relatively low rotational speeds, the conductors of the squirrel or starter cage, which generally extend axially through the rotor pole assembly from one end cap to another end cap, provide sufficient mechanical strength to hold the components of the rotor together. In many applications, however, additional support members are required. The additional support members are used to provide either radial support or circumferential support. When the added support members are of magnetic material having a radial extent, they create leakage flux paths across the permanent magnets. When the added supports are of magnetic material and have circumferential extent, they by-pass the low permeability interpolar gaps of the rotor. Either type of leakage path reduces the total useful flux density of the rotor thus reducing the total amount of torque available for a given size machine or investment in machine materials.
U.S. Pat. No. 4,139,790 to Steen discloses the use of non-magnetic strength laminations for containment. This type of design requires the use of an additional and more expensive rotor material such as stainless steel. Since non-magnetic laminations extends from the shaft to the outside of the rotor, a flux leakage path along the entire magnet width at the location of each non-magnetic lamination is created. In addition, since non-magnetic laminations extend out to the periphery of the rotor, that is, to the air gap between the rotor and the stator, the effective area for carrying flux across the air gap is reduced. Therefore, regions of the low flux concentration in the air gap are created thus decreasing the flux capability of the rotor and, reducing the ability of the rotor to generate torque. If a series of modular substacks are used, the design limits the length of the active portion of the rotor to integral multiples of the length of these substacks.
U.S. Pat. No. 4,127,786 to Volkrodt discloses the use of axially disposed bolts extending through the lamination stack and non-magnetic strength laminations for containment. In addition to the disadvantages discussed with respect to the use of non-magnetic strength laminations in the Steen patent, this requires additional parts and manufacturing steps.
Other approaches for providing strength in a rotor assembly include the use of bimetallic shrink rings, and a magnetic shunt through the magnet and rotor iron at the rotor periphery. Bimetallic shrink rings require expensive materials, are expensive to manufacture and result in non-laminated pole faces thus leading to high pole face losses. Magnetic shunts through the magnet present an iron flux leakage path along the entire machine length. In addition, machining tolerances are of the order of the dimensions of these shunts. The radial thickness of rotor iron at the rotor periphery is generally very small and comparable to machining tolerances. Such rotor iron also presents an iron flux leakage path along the entire machine length.
The above-mentioned approaches have been used successfully in machines where the mechanical and magnetic forces are extremely high. In these applications, simplicity of manufacturing and cost limitations are generally not extremely important considerations and can be sacrificed in order to obtain the desired performance.